I am glad you like the Soul 170HA, I hope the amplifier will pair well with all your headphones, I haven't try any ZMF headphone with 170HA so far, but our team in Zhuhai likes it very much.
You can use 5AR4 as a direct replacement of GZ34s in 170HA. I believe the availability of 5AR4 and GZ34s should be slightly better than the 22DE4 we used previously.
We adjusted the working current of the KT170 output tube by a few mW to achieve a different tonality. We have included a brief explanation in the Interview with Engineering video, I time-stamped the Dual Tonality feature below if you want to hear the answer from the Engineer directly.
Do you think it'd be possible to get a full list of supported rectifiers for the 170HA? I had a chat with them in Munich about it and they listed a few but I don't remember them anymore. Wondering if 274B, 5U4G etc. are possible options as well.